1. Field of the Invention
The present invention relates a plasma processing apparatus and method for forming a deposited film, particularly, a functional deposed film on a desired substrate. The functional deposed film includes amorphous semiconductor films suitable for use in the amorphous semiconductor devices such as electrophotographic light receiving members (or electrophotographic photosensitive members), image input line sensors, image pickup devices, photovoltaic devices (including solar cells), and the like.
2. Related Background Art
In the production of semiconductor devices in recent years, the so-called RF plasma CVD (high frequency plasma chemical deposition) has been frequently used. In the RF plasma CVD, there is generally used an oscillation frequency of 13.56 MHz in view of the radio regulation law. The RF plasma CVD has advantages such that the control of discharging conditions is relatively easy and a deposited film obtain has an excellent film property, but it has disadvantages such that the utilization efficiency of film-forming raw material gas is not sufficient and the deposition rate (that is, the film-forming speed) of a deposited film formed is relatively low.
There has been proposed so-called microwave plasma CVD (hereinafter referred to as MW plasma CVD) using an oscillation frequency of 2.45 GHz. The MW plasma CVD has advantages which cannot be achieved by the RF plasma CVD, such that the utilization efficiency of film-forming raw material gas is extremely high and a deposited film can be formed at an extremely high deposition rate. An experiment of such MW plasma CVD is disclosed in, for instance, Japanese Unexamined Patent Publication No. 186849/1985. Particularly, this document discloses a MW plasma CVD for film formation using a MW plasma CVD apparatus. Document 1 describes that according to this MW plasma CVD, it is possible to form a deposited film on the surface of a cylindrical substrate at a high deposition rate while effectively utilizing film-forming raw material gas at a high utilization efficiency. However, for such MW plasma CVD as described in document, there are disadvantages such that because of using a microwave energy, the density of a plasma generated upon the film formation is extremely high and therefore, film-forming raw material gas is rapidly decomposed, wherefore it is difficult to establish desired film-forming conditions capable of forming a dense deposited film.
To order to eliminate such disadvantages in the film formation by the MW plasma CVD, Japanese Unexamined Patent Publication No. 273047/1995 proposes a film-forming apparatus using an oscillation frequency of a VHF (very high frequency) range (this oscillation frequency will be hereinafter referred to as oscillation frequency).
FIG. 1 is a schematic diagram illustrating the constitution of an experiment of such film-forming apparatus using an oscillation frequency of a VHF range. In the film-forming apparatus shown in FIG. 1 comprises a reaction chamber 100 (or a deposition chamber) in which six substrate holders 105A each having a cylindrical substrate 106 positioned thereon are spacedly arranged on the same circumference so as to establish a plasma generation region circumscribed by the six cylindrical substrates 106. Reference numeral 105B indicates an auxiliary substrate-retaining member. Each substrate holder 105A is provided with a heater 140 installed therein so as to heat the cylindrical substrate 106 from the inside. Each substrate holder 105A is held on a rotary shaft 131 connected to a driving motor 132 so that it can be rotated.
The reaction chamber 100 is provided with a cathode electrode 203 for the introduction of a high frequency power into the reaction chamber 100. The cathode electrode 203 is positioned at a center of the above described plasma generation region (or at a substantial center of the circle formed by the six cylindrical substrates arranged on the same circumference). The cathode electrode 203 is electrically connected to a high frequency power source 111 through a matching circuit 209. Each of reference numerals 600A, 600B, 601A and 601B indicates an earth shield, reference numeral 107 an exhaust pipe provided with an exhaust valve and which is connected to an exhaustion mechanism having a vacuum pump, and reference numeral 116 a gas feed pipe which is connected to a raw material gas supply system 108 through a gas supply pipe 117. Reference numeral 133 indicates a vacuum-sealing member.
This second document describes that the film-forming apparatus shown in FIG. 1 in which the cylindrical substrates 106 are spacedly arranged on the same circumference and the cathode electrode 203 is arranged at a center of the space circumscribed by the cylindrical substrates is of the structure capable of attaining a high raw material gas utilization efficiency and enables formation of a high quality deposited film having a uniform thickness and a homogeneous film property on each cylindrical substrate at a high deposition rate.
Incidentally, in recent years, there have been developed various high performance instruments in which amorphous silicon (a-Si) devices (amorphous silicon device will be hereinafter referred to as a a-Si device) are used. Along with this, there is an increased demand for such a-Si devices to be more improved in terms of the quality. For instance, in the field of electrophotography, there is an increased demand for providing a high performance electrophotographic apparatus which is inexpensive, can be installed in a small space at an office, and can reproduce a highly precise image which excels in quality at a good reproduction speed. In order to satisfy this demand, it is necessary for an a-Si device (that is, an a-Si light receiving member) used in the electrophotographic apparatus to be improved.
In view of this, the present inventors conducted various studies through experiments of the conventional film-forming apparatus and methods for forming an amorphous silicon device. As a result, it was found that heat generation of the film-forming apparatus during the film formation greatly influences the operation rate and stability of the apparatus and also the quality of a deposited film formed. For instance, for the film-forming apparatus shown in FIG. 1, it was found that temperature rising due to the heat generation at the cathode electrode for introducing a high frequency power into the reaction chamber and that at an introduction portion of the cathode electrode into the reaction chamber become problematic depending upon the conditions involved.
Such temperature increase due to heat generation is liable to entail such problems as will be described, in the formation of a deposited film.
(1). The film-forming apparatus suffers from mechanical damage. Particularly, in the case of the film-forming apparatus shown in FIG. 1, the electrical isolation of the cathode electrode from the reaction chamber, an insulating member constituted by an insulating resin material such as Teflon (trademark name) or the like or a ceramic material such as alumina ceramic, boron nitride or the like is used. However, when film formation is conducted under conditions which will cause significant temperature increase at the cathode electrode or the introduction portion of the cathode electrode into the reaction chamber, there is a fear that the insulating member constituted by the insulating resin material will be melted or burned. Even an insulating member constituted by ceramic material, is liable to suffer from distortion due to thermal expansion or damage due to thermal shock. When such problems occur, there is a fear that film-forming raw material gas leaks, and when the film-forming raw material gas comprises a spontaneously flammable gas such as SiH.sub.4, such flammable gas is fired, where it is necessary to suspend the film-forming process.
(2). Spherical growth defects are liable to occur during the formation of a deposited film. Particularly, even in the case where the insulating member is not damaged as will be described in the above, when the temperature of the cathode electrode is significantly raised, the stress of a film deposited on the surface of the cathode electrode during the film formation is gradually increased as the temperature of the cathode electrode is raised, where the film deposited on the surface of the cathode electrode is sometimes peeled off.
It is known that when such film-peeling occurrs during the film formation, the peeled film generates particles, which deposit on the surface of a deposited film formed on a substrate, where abnormal film growth based on nucleuses comprising such film particles occurs to provide spherical growth defects in the deposited film formed on the substrate.
In the case where such spherical growth defects occur in the surface region of an a-Si deposited film, when an a-Si device as an electrophotographic light receiving member is prepared using such amorphous silicon deposited film, the electrophotographic light receiving member is liable to entail problems such that a minute white spot appears on an image reproduced, or a minute black dot appears on an image reproduced in the case of reverse development.
For an electrophotographic apparatus having a markedly improved performance in recent years, when the electrophotographic light receiving member used therein comprises an a-Si electrophotographic light receiving member (an a-Si device) containing a spherical defect which could be disregarded in the past, the spherical defect sometimes appears in the form of a minute white spot on an image reproduced. In this connection, it is necessary for the a-Si device to be improved in terms of the quality.
From the background as above described, an appropriate temperature-controlling mechanism is necessary to be provided for the introduction of a high frequency power and for the structure of the cathode electrode in the film-forming apparatus shown in FIG. 1. But the prior art is absolutely silent about this.